In art, this invention is believed to be in Class 361, Subclass 42.
This invention relates to an amplifier circuit for a current transformer which has a low DC circuit impedance but a relatively high AC circuit impedance. Such current transformers approximate constant-current AC sources when operated at frequencies within their operating frequency range.
Normally, current transformers are used to measure fairly large AC currents, from one or two amperes up to 1000 or more amperes. A typical current transformer has a ratio in the range of 10:1 to 1000:1. Under these conditions, the output signal from the current transformer is quite large and amplification is unnecessary. However, in certain specialized applications, such as ground fault detection, it is necessary for current transformers to measure AC currents as small as 1 or 2 milliamperes. Under these conditions, amplification of the signal from the current transformer becomes necessary.
When using a current transformer to measure very small currents a difficulty arises from the characteristics of the current transformer itself. Current transformers are most accurate when terminated into a low impedance load; the lower the impedance the more the current transformer acts like an ideal transformer with a turns ratio that remains constant at all currents. This is especially true when attempting to measure very low currents. However, when trying to measure a very low current, with a very low impedance across the current transformer output, the resulting signal output is very, very small, and it must be amplified to bring it up to a useful value. With conventional amplifiers, this causes all sorts of problems with noise, amplifier drift and other effects which adversely affect the accuracy of the current measurement.
The object of the invention is an improved amplifier circuit which provides high amplification and presents a very low impedance to the signal source. An additional feature of the improved amplifier circuit is the ability to detect an open circuit or a short-circuit to ground in the current transformer or its associated wiring. When used in conjunction with an ordinary iron-core current transformer, this amplifier circuit allows accurate current measurements down to a fraction of a milliampere.